ARM: dts: imx: use generic node name for rave-sp

Use generic "mcu" node name for rave-sp node, as recommended by
Devicetree specification.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>